What you've described is where the rubber meets the road. Actually living through what you've said, theoretically, you can handle.
I just started being poly about 6 months ago. My significant other has been poly for years and it was something I had to be on-board with if I was going to date her.
As a small backstory, we were with each other non-stop for about two months before she went out on her first date with someone else. When she finally told me, it put my stomach in knots and had me pacing the floorboards.
Since then, she's gone on at least 7 or 8 dates (some repeat dates with the same guy, but nothing continuous) and I've been able to get to a headspace where I have even driven her to the dates, picked her up, and even helped her get dressed.
My roommates had to spend some time with me while she was out on her dates, in essence holding my hand because I was on pins and needles. Part of that was because I had no way of knowing when she was coming home to me and that was nerve wracking. For me at least, it has gotten progressively easier.
That said, we've just been through a stage where she's been with me 24/7 for over a month. I can tell that she's got some new friends on Tinder and OkCupid so it won't be long until she goes on another date. And I do wonder if one of them will be a new steady boyfriend in addition to me. That next person could always be "the one" who really impacts your relationship.
The best advice I can give you is that you need to get your own stuff going on in life and try to become as independent as you can. Take the relationship for what it's worth and what you get. Don't be afraid to voice if you aren't getting what you need out of the relationship, but don't make it an ultimatum. Either you're getting what you want or you aren't. Once your partner knows that, they can either decide they want to adjust their behavior or they don't. It's as simple as that.
I wish you both well. I know it's not easy.
Me - male, 42, poly, straight, in a serious relationship with Audrey, also casually dating.
Audrey - female, 20, poly, pansexual, also casually dating.